WIP: Direct Solver

This is the last big piece of the direct solver: the actual skeletonization and recursive compression. It's still a bit of a WIP, but it seems to mostly works as expected. The big chunks of the code are in linalg/hss.py and tests/test_linalg_hss.py.

• Needs some more docs and cleaning up.
• API? There's a build_compressed_matrix function that returns a class that has a matvec method so that it behaves like scipy's LinearOperator.
• None of it works with vector densities or multiple domains or anything fancier like that.